The present invention relates to a method of and an apparatus for bleaching medium consistency, the consistency being from 5 to 25%, preferably from 10 to 15%, pulp. The method of the invention is particularly well applicable in applications where the volume of the gas used for the bleaching is larger then in the conventional ozone bleaching e.g. exceeding 2 m3/adt.
Many prior art publications on bleaching of medium consistency pulp are already known, the first of which is European patent application no. 397308 of A. Ahlstrom Corporation. The invention has resulted in several mill-scale applications, already. Said EP application describes for the first time in detail how ozone bleaching can be effected at the medium consistency range in a way acceptable in industrial mill-scale operation. According to the publication, pressurized gas consisting mainly of carrier gas and ozone is mixed in a fluidixing mixer into pulp so as to produce a foamy suspension of liquid, gas and fibers, the suspension being transported from the mixer to a reaction vessel which may be a larger vessel or, for example, a portion of the flow channel subsequent to the mixer. After the ozone reaction, residual gas is separated from the suspension, said gas consisting mainly, as is known, of the carrier gas mixed with the ozone into the pulp. If it is desirable to introduce a larger amount of ozone into the pulp, it is possible according to the publication to employ two or more fluidizing ozone mixers for example by introducing the pulp subsequent to the first ozone reactor and the gas separation to a second fluidizing mixer in which another dose of pressurized mixture or ozone and carrier gas is mixed into the pulp and from which the pulp is directed further to a second reactor, etc.
A German patent application DE-A-40 39 099 discloses medium consistency ozone bleaching in combination with chlorine dioxide. The main goal of the method is to effect both the ozone and chlorine dioxide bleaching simultaneously without intermediate washing. The publication discusses also an optional method of mixing ozone with pulp by means of several mixers connected in series. Said method is similar to the one described in the above mentioned EP patent application i.e. fresh ozone is introduced into each mixer and mixed thereby with pulp.
Also WO publication no. 93/00470 is known which discloses a bleaching method different from the method described above. Also according to this publication the mixture of ozone and carrier gas is mixed into the pulp by a fluidizing mixer and the mixture of gas and pulp is introduced under pressure to a reaction vessel. In the upper section of the reaction vessel, gas is separated from the pulp and additional chemicals, such as sodium hydroxide, hydrogen peroxide or chlorine dioxide, are added to the pulp. After this the pulp flow is introduced to a second reaction vessel in which the pressure is maintained at a clearly lower level than in the first vessel. Gas is separated from the pulp also in the upper section of the second vessel from which it is transported to a separate further treatment or for other use.
Even though the apparatus described above already function in mill scale use they still have a few drawbacks, for example if it is desirable to improve the bleaching efficiency without increasing the size of the reaction vessel. For example, the method of the EP publication mentioned above requires, a separate reaction vessel and a separate gas separation apparatus for each ozone mixing stage, etc.
Also, it has been found out that in mill-scale experiments the amount of residual ozone in gas separation exceeds acceptable limits being in some applications more than 50 percents. Naturally, the higher is the amount of residual ozone the less is the brightness of the pulp improved in the ozone bleaching stage. One of the reasons for this high waste of ozone is the fact that the residence time of pulp in a mixer is not sufficient for ozone bleaching, especially, if the amount of ozone is high whereby ozone is, after the first fluidization stage, in the form of substantially large bubbles. One solution to the above addressed problem is discussed in the above mentioned EP patent application EP-A-0 397 308 i.e. dividing the ozone charge to several smaller ozone charges whereby the fluidizing mixer is able to mix the smaller amount of ozone more effectively and more evenly.
One of the objects of the present invention is to find another solution to the problem which reduces the equipment requirement and improves the bleaching efficiency of a pulp mill compared to conventional methods. The method of the invention allows the use of a larger gas dose, resulting either in the use of larger ozone volumes or in that use of weaker ozone mixtures in the bleaching. Compared with the solution of EP-A-0 397 308 only one ozone introduction pipe for the entire bleaching stage is needed resulting in reduced risk of ozone leaks in the mixers.
Another object of the invention is to find a solution to the problem relating to the excess waste of ozone. The method of the invention extends the efficient treatment time with ozone by means of arranging several fluidizing mixers in series to fluidize the pulp, ozone and carrier gas mixture the ozone and carrier gas being introduced into pulp in the first one/s of said fluidizing mixers. It is a characteristic feature of the invention that there is at least one fluidizing mixer in said series of mixers in which only unreacted ozone from a previous mixer is remixed with pulp.